The invention in general relates to nuclear magnetic resonance (NMR) spectroscopy, and in particular to a low-temperature radio-frequency coil for an NMR probe.
Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and special-purpose radio-frequency (RF) coils for generating time-varying magnetic fields B1, perpendicular to the field B0. Each RF coil resonates at the Larmor frequency of a nucleus of interest. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in test tubes or flow cells. The direction of the static magnetic field B0 is commonly denoted as the z-axis, while the plane perpendicular to the z-axis is commonly termed the x-y plane.
The sensitivity of an NMR spectrometer can be limited by its RF coils. In particular, conventional room-temperature copper coils can have limited Q-factors, which can limit measurement sensitivities. High-temperature superconductor (HTS) coils have been proposed as alternatives to conventional room-temperature coils. HTS coils allow higher Q-factors than conventional room-temperature coils. Typical HTS materials have relatively high diamagnetic susceptibilities, however, and thus can significantly disturb the uniformity of applied external magnetic fields. Magnetic field non-uniformities lead to line-broadening, and can limit measurement sensititivities. Furthermore, conventional HTS materials may not support the RF currents required for some NMR experiments.
The present invention provides low-temperature radio-frequency coils and systems for nuclear magnetic resonance applications. In particular, the present invention provides a nuclear magnetic resonance spectrometer comprising a magnet for applying a static magnetic field B0 to a nuclear magnetic resonance sample, and a nuclear magnetic resonance probe comprising a susceptibility-compensated cryogenic normal metal radio-frequency coil for applying a radio-frequency magnetic field to the sample. The radio-frequency coil includes an internal normal metal layer, and a pair of external normal metal layers cladding the internal layer. The internal layer has a first magnetic susceptibility and a first conductivity at the operating temperature. The pair of external layers have a second magnetic susceptibility opposite in sign to the first magnetic susceptibility, and a second conductivity that is preferably higher than the first conductivity. In the preferred embodiment, the external layers are made of pure aluminum while the internal layer is made of copper.